Field of the Invention
The present invention relates to a method for manufacturing an ultrathin organic light-emitting device, and more particularly, to a method for manufacturing an ultrathin organic light-emitting device, of which the light extraction efficiency can be improved and the thickness can be significantly reduced.
Description of Related Art
In general, light-emitting devices can be generally divided into organic light-emitting devices in which a light-emitting layer is made of an organic matter and inorganic light-emitting devices in which a light-emitting layer is made of an inorganic matter. In these light-emitting devices, the light-emitting layer is a self light-emitting layer which generates light using energy emitted from excitons that are generated through the recombination of electrons injected through a cathode and holes injected through an anode. Such organic light-emitting devices have a variety of advantages, such as, low-voltage driving, self-emission, a wide viewing angle, a high resolution, natural color reproduction and rapid response.
Recently, active studies are underway in order to apply organic light-emitting devices to a variety of devices, such as portable information devices, cameras, watches, office equipment, information display windows of vehicles, televisions (TVs), displays, or illumination systems.
Approaches for improving the luminous efficiency of organic light-emitting devices include an approach of improving the luminous efficiency of a material that constitutes a light-emitting layer and an approach of improving the light extraction efficiency at which light generated from the light-emitting layer is extracted.
The light extraction efficiency depends on the refractive indices of layers which form an organic light-emitting device. In a typical organic light-emitting device, when a ray of light generated from the light-emitting layer is emitted at an angle greater than a critical angle, the ray of light is totally reflected at the interface between a higher-refractivity layer, such as a transparent electrode layer, and a lower-refractivity layer, such as a glass substrate. This consequently lowers the light extraction efficiency, thereby lowering the overall luminous efficiency of the organic light-emitting device, which is problematic.
More specifically, only about 20% of light generated from the light-emitting layer is emitted to the outside and about 80% of the light is lost by a waveguide effect originating from the difference in the refractive index between the light-emitting layer which includes an electron injection layer, an electron transport layer, an emissive layer, a hole transport layer, and hole injection layer, and an anode, and a glass substrate, as well as by a total internal reflection originating from the difference in the refractive index between the glass substrate and the air. Here, the refractive index of the internal organic light-emitting layer ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO) which is generally used for the anode is about 1.9. Since the two layers have a very small thickness ranging from 200 to 400 nm and the refractive index of glass used for the glass substrate is about 1.5, a planar waveguide is thereby caused inside the organic light-emitting device. It is calculated that the ratio of the light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate is about 1.5 and the refractive index of the ambient air is 1.0, when the light is directed outward from the inside of the glass substrate, a ray of the light having an angle of incidence greater than a critical angle is totally reflected and is trapped inside the glass substrate. The ratio of the trapped light is up to about 35%, and only about 20% of the generated light is emitted to the outside.
The organic light-emitting device is gaining increasing interest due to its slim profile that is thinner than those of the other light-emitting devices, such as light-emitting diodes (LEDs). Since the self-emitting organic light-emitting device does not require a backlight unit (BLU) to be added thereto, the organic light-emitting device advantageously allows a final product with a thickness of several millimeters. However, although it is desired to further reduce the thickness of the organic light-emitting device, it is impossible to further reduce the thickness of the organic light-emitting device based on the existing structure or process, since there is a limit in reducing the thickness of a glass substrate or an encapsulation glass substrate of the organic light-emitting device. When the organic light-emitting device is fabricated under the existing process, the minimum thickness of the glass substrate is 0.1 mm. When the organic light-emitting device is fabricated based on the existing process, the thickness of the organic light-emitting device must be greater than the thickness of the glass substrate.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.